Reynolds Number Calculator
Calculate the Reynolds number and determine flow regime for pipe flow.
Results
The Reynolds number predicts whether fluid flow will be smooth (laminar) or chaotic (turbulent). Below Re = 2300, flow is laminar with predictable pressure drop. Above Re = 4000, flow is turbulent with higher friction but better mixing. The transition zone between 2300 and 4000 is unpredictable. This is one of the most important dimensionless numbers in fluid mechanics.
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Frequently asked questions
Re = 998 × 1 × 0.05 / 0.001 = 49,900. This is well into the turbulent regime (above 4000). Most plumbing and piping operates in turbulent flow.
Flow transitions from laminar to turbulent between Re 2300 and 4000. Below 2300 is always laminar, above 4000 is always turbulent. The transition zone is unstable and unpredictable.
Higher viscosity reduces Re, promoting laminar flow. Honey (µ ≈ 10 Pa·s) at 1 m/s in a 50 mm pipe has Re = 5 - deeply laminar. Water (µ = 0.001) at the same conditions has Re = 49,900 - turbulent.
Turbulent flow has higher friction losses, requiring more pump energy. For laminar flow, pressure drop is proportional to velocity. For turbulent flow, it is proportional to velocity squared.
Yes, use the hydraulic diameter: Dh = 4A/P where A is cross-sectional area and P is wetted perimeter. For a square duct, Dh equals the side length. The same Re thresholds (2300/4000) apply.